The present invention relates generally to a system and method for treating a metal strip with reactive chemicals and more particularly to a system and method for pretreating running webs of metal prior to coil coating.
It will be appreciated by those skilled in the art that metals must be cleaned of protective films such as oils and then pretreated in some manner in order to provide adhesion and corrosion resistance to the finished product. Although it is easy and inexpensive to treat a single sheet of metal, coil coating is performed at relatively high speeds. Therefore, the strip must be pretreated at similar speeds. In the past, coating has been performed by having a multi compartmented system having a portion of which pretreats the running coil of metal in a pretreatment bath. Unfortunately, this prior system has several drawbacks. The fluid is usually a reactive species which generates reaction by-products, "sludge", when interacting with the coil. Sludge building up in the bath often works its way into comers and other nooks and crannies. Therefore, when the pretreatment solution is changed out, the sludge must be physically removed from the nooks and crannies after it has been given a chance to accumulate. Further, metal web running at a relatively high rate of speed is immersed or submerged in the pretreatment bath for only a certain period of time. Lengthening the amount of time in the bath slows down the process. However, shortening the amount of time that the running web is submerged in the bath reduces the efficiency of the pretreatment thereby allowing sludge material to remain on the running web thereby decreasing the ability to coat the material. Also, the sludge created in the system that is left in the system for a long period of time continues to react with the pretreatment fluid thereby reducing the amount of pretreatment fluid that can actually react with the running web. Further, sludge remaining in the pretreatment fluid as it continues to react with the pretreatment fluid reduces the life span of a given amount of pretreatment fluid thereby increasing the amount of fluid required and increasing the amount of fluid that must be taken care of pursuant to EPA requirements. Further, the prior art provided that the only method of controlling the bathing time was to increase or decrease the web speeds and consequently alter the immersion time.
What is needed, then, is a treatment system that incorporates by design the ability to modify the immersion time very quickly and easily. This needed system and method must also provide for a high mass transfer of reactive species to the strip. This needed system and method must eliminate sludge build up and allow for a rapid change out of one type of chemical treatment to another. This needed system and method must also reduce the amount of chemical that is actually used and that must be disposed of. This needed system and method must reduce the amount of bathing time required by the web. This needed system and method must increase the amount of materials removed from the surface of the metal web. This needed system and method is presently lacking in the prior art.